I have a newly purchased 52 mercury. It has a 77 firebird front and rear suspension with front disc and rear drums. It has a 8" dual diaphragm power booster mounted to the firewall at the original master cyl location and a dual master cyl. that looks like possible a gm type. The brake pedal stops the car but is near the floor. I can disconnect the vacuum line to the booster and the brake pedal is near the top of travel and stops the car fine. Reconnect the booster and pedal back near floor. I replaced the booster with a new one same result. I have 22" of vacuum on the line coming from engine to the booster. There is a Proportioning Valve installed in line. I have made sure the brake pedal push rod is correctly adjusted as well as the booster to master cyl rod. I'm open to any suggestions. I have never run into this type of issue. It has the original 52 mercury brake pedal so has quite a bit of travel available, so a lot of difference between near the floor and at the top of travel.
Larryb My name is Dave, I would like to see if I can help you and I am an old time mechanic and have done a lot of brakes. If you would be comfortable, give me a call. I feel it is easier to diagnose problems verbaly than by E mail. 818 445 0431 Dave call any time.
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